Migrate from the `failure` crate to `anyhow`

Migrate from the `failure` crate to `anyhow`

The `anyhow` crate interoperates with the `std::error::Error` trait
rather than a custom `Fail` trait, and this is the general trend of
error handling in Rust as well.

Note that this is mostly mechanical (sed) and intended to get the test
suite passing. As usual there's still more idiomatic cleanup that can
happen, but that's left to later commits.